Tap



A ril 19, 1938. Q. E. KOEHLER ET AL 2,114,448

TAP

Filed Nov. 10,1956

Patented Apr. 19, 1938 TAP Oscar E. Koehler and Earl R. Koonz, Greenfield, Mass., assignors to Greenfield Tap and Die Corporation, Greenfield, Mass, a corporationofMassachusetts Ap lication November 10, 1936, Serial No. 110,063

1 Claims. (01. 10-441) This invention relates to taps and more especially to taps having relatively wide lands.

When holes to be threaded have key-ways or slots, the lands of the tap used obviously must be wide enough to span the key-way or slot, and when the tap is of large diameter, wide lands are necessary to hold it in position and prevent chattering while the thread is being cut. It has been found, however, that excessive friction is developed in using such taps and various expedients have been proposed for reducing it.

A conventional solution of this problem is to relieve the thread by progressively deepening and widening the grooves between'the threads from or near the cutting edge to the heel of each land. While this procedure is effective in reducing the friction, it is found'when'the' rotation of the tap is reversed to remove it from the hole, that chips often lodge in the relieved or recessed portions of the lands and are dragged into the threads which have just been cut, thereby impairing or completely destroying the finished work.

The object of this invention is a tap of simple and inexpensive construction which is capable of cutting a clean thread without excessive friction, and which may be readily removed without injury to the work.

We have found that the friction is minimized by a special reduction of the central portion of the lands, leaving both the heel and the cutting edge at full pitch diameter. This gives the desired relief and at the same time avoids objectionable wedging on reversal because chips cannot enter the relieved portion during the reverse motion of the tap.

In the drawing,

Fig. 1 is a perspective view of a tap according to this invention.

Fig. 2 is an enlarged cross-sectional view of the tap showing the relieved portion of the lands.

Fig. 3 is a plan view of a portion of one of the relieved lands.

Fig. 4 is a sectional view of the land along the root of the threads shown in Fig. 3.

Fig. 5 is an end view of the portion of the land shown in Fig. 3, and

Figs. 6 and '7 show a relieved land of an alternate construction.

The tap of Fig. 1 comprises a shank II with a plurality of threaded lands l2 separated by flutes l3 and tapered at the end [4 in the usual manner. The section in Fig. 2 taken at the root i5 of the thread shows the relieved portions l6 extending over the central parts of the lands between the cutting edges l8 and the heels l9 and converting the arc of the root of the thread into substantially a chord 20, as shown more clearly inFig;4'. 5

, In most cases, suflicient relief is obtained by merely reducing the width of flat portions 2| of the threads without cutting them away entirely, so that thecrest 22 of the threads remains concentric and there is no substantial reduction of thread diameter even at the center of the land where the relief is a maximum. At both the cutting edge and the heel the cross section of thread is unchanged, as' shown in Fig. 5, but at the center of the relieved'portion the flanks of the threads will assume the shape indicated by the dotted line 23.

Alternately, the relieved area may be of the shape shown inllig. '7, in which the convex'arc at the root of thejthread is converted into a slightlyconcave are 24', but asin the previous casethe'flats 2| of the thread are preferably not reduced beyond the point where the crests come to a sharp edge 25 of full pitch diameter.

In making the tap of this invention, the grinding wheel is set to grind the lands on the correct pitch diameter and is then given a predetermined inward shift cutting into the root circle as the wheel approaches and recedes from the center of each successive land so as to relatively reduce the flanks over the central part of the lands and convert the are at the root of the thread to a chord or to a concave are as the case may be. Or the threads may be ground in usual manner and the grinding wheel passed through the thread in separate operation, giving the desired cutting action at the centers of the lands.

The type of relief shown in Figs. 6 and 7 may be obtained by rotating the tap intermittently and moving the grinding wheel 26 radially of the tap into the groove between the threads to the position shown in Fig. 7 while the tap is at rest. The wheel is then retracted beyond the root circ-le 21 and the tap is rotated to bring the next land into grinding position. This procedure is repeated until the tap is relieved throughout the full length (or any desired portion) of the thread, it being understood that the wheel is being continuously advanced under the control of the lead screw, as in the ordinary grinding operation.

Other methods of grinding this chordal type of relief will be obvious and will vary in the shapes of the parts ground away and in the conformation of the bottom surface of the groove, the latter being either concave as shown in Figs. 4 and 7 or substantially fiat or convex, depending upon the particular method used.

The tap of this invention lends itself readily to accurate formation during the grinding of the tap threads, the grinding tool simply being moved with relation to the threads to give the desired relief. The resulting construction preserves the full thread formation at each end of the land and at the same time obviates the objectionable frictional drag and substitutes for this a helpful distribution of the lubricating oil. The central portions of the lands cut away in accordance with this invention provide extended shallow pockets on each side of each thread catching the lubricating oil and carrying it and spreading it over the freshly-cut surfaces of the threads as they are formed by the tap. This relieving of the central portions also maintains the strength and rigidity of the threads and preserves an accurately centered continuous connection between the cutting and trailing portions so that the full strength of the entire thread is available in support of the cutting edge in each direction.

While this invention has been described in connection with specific embodiments, it is not limited thereto. Other methods of providing the relief may be used and cam control of the grinding wheel may be employed to give any desired form to the out including, for instance, a precisely straight lined chord 20 at the root of the relief.

We claim:

1. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands, the threads on a portion of each land having crests diminishing in width from both the cutting edge and the heel toward the center of the land.

2. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands, and with threads on each land having intermediate portions diminishing in width from both the cutting edge and the heel toward the center of the land.

3. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands, and with threads on each land having intermediate portions diminishing in width from both the cutting edge and the heel toward the center of the land while maintaining the apexes of said threads undiminished in height.

4. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands and with threads on each land diminished in width at intermediate portions between the cutting edge andthe heel, the grooves between the threads being deepened adjacent the intermediate thinner portions of the threads.

5. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands, and with threads on each land having intermediate portions ground to diminished width from both the cutting edge and the heel toward the center of the land while maintaining the apexes of said threads undiminished in height.

6. A tap havingthreaded lands separated by flutes defining cutting edges and heels on the lands, and with threads on each land ground to diminished width at intermediate portions between the cutting' edge and the heel, the grooves between the threads being deepened adjacent the intermediate thinner portions of the threads.

7. A tap having threaded lands separated by flutes defining cutting edges and heels on the lands, and with threads on each land ground to diminished width at intermediate portions between the cutting edge and the heel, the grooves between the threads being deepened on lines of concave curvature adjacent the intermediate thinner portions of the threads.

OSCAR E. KOEHLER. EARL R. KOONZ. 

